1. Field of the Invention
The present invention relates to an image reading apparatus for electrically reading image data and, more particularly, to an image reading apparatus for forming image signals which reflect original image well.
2. Description of the Prior Art
Apparatuses have been recently devised and used which read original images utilizing solid-state image sensors such as CCDs, convert the obtained electrical analog signals into digital signals, and reproduce images with a printer or transmit the signals to a remote location.
In an apparatus of this type, an image signal correction circuit is frequently incorporated so as to electrically correct the image signals to eliminate the adverse effects of irregularities in the light emission by a fluorescent lamp, light emission variations in an optical system, or sensitivity fluctuations of the CCD, thereby stabilizing the operation. When an image signal correction circuit of this type is designed especially for an apparatus which handles high-speed image signals, expensive circuit elements must be used to allow high speed operation. For this reason, when complex correction processing or high precision signal processing is to be performed, the circuit size must be increased and the cost becomes higher.
An image signal correction circuit is known in which an image signal corresponding to a standard white plate is stored in a memory or the like as a digital correction signal, and the image signal corresponding to an original is corrected in accordance with this correction signal. In this circuit, when high-speed processing is performed, a high level of noise is introduced in the correction signal stored in the memory or the like. In one method of eliminating such noise, an average value of adjacent pixels is calculated.
For this purpose, extra circuit elements such as adders, multipliers, or timing circuits for controlling the operation timings of the respective circuit elements are required. Such circuit elements, moreover, must be able to operate at high speed and are therefore expensive.
In an apparatus having such an image signal correction circuit, even if a read image has some defect, there is no method other than producing a hard copy with a printer or the like to check the image quality. For this reason, detection of defects in the image quality is time-consuming. This has decreased productivity at factories and degraded maintenance and other services.
In an apparatus of the type described above, the amount of light from an illuminating means is controlled so as to reproduce a read image at a constant density level.
However, if a fluorescent lamp or the like used as the illuminating means is burnt out or is degraded in quality or the light regulating circuit becomes abnormal, a satisfactory method for detecting such as defect has not yet been established, and detection of such a defect is thus time-consuming.
According to a general light regulating method adopted in such an apparatus, an electrical signal corresponding to a standard white plate is obtained and is sampled at a specific timing in each frame, and the light regulation of the illuminating means is performed based on the thus obtained density data.
However, the fluorescent lamp used as the illuminating means has an irregular luminous intensity distribution. Therefore, if the light regulation is performed in accordance with density data which is obtained at such a specific timing, the electrical signal may saturate in the remaining portion of the signal frame, or the amount of illuminated light may become insufficient. In either case, satisfactory light regulation cannot be performed. Furthermore, when light regulation is performed with the fluorescent lamp in the cold state, the lamp cannot be turned on immediately and satisfactory light regulation cannot be performed.
In an apparatus of this type, when the tube wall temperature of the fluorescent lamp deviates from the suitable operating range, it becomes difficult to turn on the lamp and the luminous efficacy of the lamp is decreased. When image reading is performed under such conditions, the obtained image quality is degraded and flickering noise may be included in the read image.
In order to determine if a normal image is being reproduced in an apparatus of this type, a printer or the like must be actually connected to the apparatus so as to check with a hard copy. In order to allow easy monitoring on an oscilloscope, a pattern must be generated repeatedly. For this purpose, a special test pattern generator is externally connected to the apparatus.
Detection of a fault is a time-consuming operation if such a test pattern generator is not available.
In an apparatus of the type described above, a fluorescent lamp is most frequently used for illuminating an original due to its good luminous efficacy. However, when a fluorescent lamp is used in practice, the tube wall temperature must be maintained within a predetermined range so as to allow efficient light emission. Since the light amount distribution of the tube is not uniform, so-called shading correction must be performed. In addition to this, since the amount of light of the lamp changes over time and depends upon the total or cumulative ON hours, the light amount control must be performed so as to allow stable image reading.